1. Field of the Invention
This invention relates in general to medical needles, and more particularly, to medical needles having enhanced echogenicity.
2. Description of Related Art
During the past several decades, ultrasonic imaging techniques have become increasingly prevalent in clinical diagnoses, and more particularly in obstetrics, gynecology and urology. Specialists in these disciplines use reflected ultrasound wave energy to image a wide variety of medical abnormalities, including malignant and benign cysts and tumors and fetal status in utero, as well as "real-time" monitoring of needle location during such procedures as fetal blood sampling, amniocentesis, tissue aspiration biopsy and core biopsy. Considerable effort has been expended to significantly enhance the ultrasound image of a needle, or at least its point or tip, in order more accurately to pinpoint its placement during real-time ultrasonic guidance. Not only is accurate guidance required to obtain the proper sample, but it is also necessary to avoid puncture or damage to tissues.
The term echogenicity refers to the relative extent that a surface reflects incident ultrasound wave energy directly back to a sensor, which is proximal to the source or emitter of the ultrasonic wave energy. The low practical echogenicity of a smooth cannula hampers accurate imaging of the cannula within a patient's body. When the smooth cannula is oriented at right angles to the ultrasound waves, the ultrasound waves are directly reflected off the cannula back to the ultrasound transducer, and the cannula is said to have a relatively high practical echogenicity. At other orientation angles, less of the ultrasound energy is directly reflected back to the transducer reducing the practical echogenicity of the cannula.
Prior biopsy needle designs have attempted to increase the echogenicity of the cannula by presenting a roughened outer surface or diffraction grating on the cannula. However, this has the disadvantage of increasing trauma to the body tissue as the cannula is inserted, causing increased discomfort to the patient.
For instance, U.S. Pat. No. 4,401,124, issued Aug. 30, 1993 to Guess et. al., outlines some of the problems associated with monitoring the insertion and guidance of needles and other instruments using ultrasound imaging. The Guess et. al. patent also discloses a proposed solution to the monitoring problem by providing, in an ultrasound pulse-echo imaging system, a diffraction grating disposed on the surface of the surgical instrument. The diffraction grating is disclosed to have a specified distance between the depth of adjacent grooves, that distance D being a function of various parameters including the center wavelength .lambda.o of the transducer and the angle .theta. between the incident beam and a line along the surface of the instrument perpendicular to the grooves.
U.S. Pat. No. 4,869,259, issued Sep. 26, 1989 to Elkins, discloses a surgical instrument such as a needle which is particle blasted to produce a uniformly roughened surface portion for use with an ultrasound imaging system to provide real-time monitoring of the location of a specific portion of the needle during insertion and guidance inside the patient's body. U.S. Pat. No. 4,977,897 issued Dec. 18, 1990 to Hurwitz, teaches a similar approach where, in addition to the roughened surface portion, the needle has one or more sounding apertures formed thereon. The diameter of each sounding aperture is substantially equal to a predetermined wavelength of an incident ultrasonic wave energy beam.
Great effort has been expended to produce a biopsy needle with enhanced echogenicity; however, any protrusions or roughness on the outer surface of the cannula greatly enhances the trauma experienced by the patient during insertion of the cannula into the patient's body. One attempt to overcome this limitation or disadvantage is described in laid-open German Patent Application No. 2,425,724 in the name of Siemens AG, wherein the outer surface of the entire cannula disclosed is coated with a thin Teflon layer for reducing the puncture friction resistance of the cannula. However, the outer surface of the disclosed cannula is still quite rough due to the echogenicity enhancing ridges on its outer surface.